1. Field of the Invention
This invention relates to temporal post-processing of compressed moving images and processes for improving quality of compressed moving images having low frame rates.
2. Description of Related Art
Phone lines are commonly used for digital communications between computers or other devices using modems and standard communications protocols. Such communication protocols have bit rates limited by the quality of transmission over the phone lines. For example, the V.FAST and H.26P standards for PSTN lines have bit rates between 16.8 and 28.8 kbit/s depending on the quality of the connection. These bit rates are low when compared to the bandwidth needed for transmitting high quality digital moving images, especially if the bandwidth also carries audio and/or other information.
Conventional moving images are a series of frames (or still images) which are displayed sequentially. The frames can be represented digitally by two-dimensional arrays of pixel values which indicate colors and/or intensities of pixels in the frames. Transmission of uncompressed pixel values by videophones is impractical because of the large amount of data required to transmit every pixel value in every frame of a moving picture. Accordingly, videophone systems contain encoding circuits which compress a series of two-dimensional arrays of pixel values into codes representing the moving image and decoding circuits which convert codes back into a series of two-dimensional arrays.
Frame difference coding such as DPCM (differential pulse coded modulation) is a well known compression technique that removes redundant information from a representation of a moving image. Frame difference coding subtracts pixel values of a preceding frame from pixel values of a current frame and extracts non-zero values which indicate changes between the frames. Redundant data, data repeated in successive frames, appear as zeros in the difference frame, and the large number of zeros can be efficiently coded or removed. Motion estimation techniques further reduce the number of non-zero values by subtracting from each block in a current frame, a block in a preceding frame at a position indicated by a motion vector. The motion vector is selected to reduce or minimize the difference.
Even with compression techniques, the bit rate of a communication channel limits the maximum frame rate, number of frames per second, for a moving image. For example, the H.26P standard limits video transmission to less than ten frames per second. At low frame rates, motion in displayed moving images appears jittery or discontinuous rather than smooth. Accordingly, processes for improving moving image quality at low bit rates are widely sought.